Optical fibers are used in a wide range of applications as a medium for delivering laser light energy to destinations remote from the light source. Perhaps the most common use for fiber optics is in telecommunications. Until recently, most optical fiber applications required the transmission of relatively low power. For example, the light power transmitted in telecommunication applications is generally 0.010 W. At present, there is a growing need to transmit higher amounts of power through optical fibers. For example, the transmission of high power laser light is required in many medical laser applications, in electronically safe firing mechanisms for triggering explosives, for use in the optical triggering of spark gaps, in high-powered optical sensors and in the remote generation of shock waves in materials. These applications generally require the transmission of &gt;10 W of power through the optical fiber.
The amount of power that can be transmitted by an optical fiber is primarily a function of the fiber's cross-sectional area. An optical fiber has a cross-sectional area of .pi.d.sup.2 /4. Thus, the power handling capability of a fiber is related to the square of the fiber diameter (d) or core size. For example, a 400 .mu.m core-fiber should theoretically transmit 16 times as much power as a 100 .mu.m core-fiber. Thus, as the use of high powered laser light applications grows, the utilization of large core fibers will increase in importance.
Independent of the size of the fiber employed, the amount of power that can be transmitted by an optical fiber is primarily limited by the quality of the input and exit faces of the fiber. Irregularities in the fiber faces and subsurface defects in the fiber can cause the scattering of laser light within the fiber resulting in damage to the fiber. Thus, the amount of power that can be transmitted through a fiber can be enhanced by minimizing the number of surface irregularities and subsurface defects present in the fiber.
High quality fiber faces can be achieved either by cleaving the optical fiber or by polishing the optical fiber face. Before cleaving or polishing the fiber, the jacket and buffer materials on the fiber are removed from the fiber end. The bared fiber is then cleaned to prevent later contamination of the cleaved or polished fiber face.
Polishing is one method for preparing fibers with high quality fiber face surfaces. In order to polish a fiber, the fiber is first mounted to a connecter by either an adhesive or by crimping. The fiber is then cut or cleaved close to the connector to minimize the amount of material that must be removed. The connector is then inserted into a holding fixture which holds the connector tight and perpendicular to the polishing or lapping surface. The fiber is first polished with a course grit lapping paper, (30 micron grit), with which the fiber is ground until it is almost flush with the connector. Grinding can be accomplished by hand, by drawing the fixture across the lapping surface in a figure eight pattern, or by using a rotary polishing wheel. Polishing is continued with incrementally smaller lapping grit papers until a mirror finish is achieved with 0.3 to 0.1 micron grit paper.
Polishing is an inefficient means for achieving high quality fiber face surfaces. Polishing is time consuming, taking roughly one hour per connector. Polishing is also labor intensive and generally requires the use of trained polishers. Polishing must also be performed with care to prevent the formation of subsurface defects in the fiber. The lapping compounds or films used also increase the cost of preparing a fiber end.
The quality of a polished fiber face can be quantitatively evaluated according to its damage threshold. The damage threshold of a fiber corresponds to the ability of the fiber end surface to withstand laser power densities at or near the damage threshold of the bulk material. The damage threshold of bulk material has been estimated at 2.4.times.10.sup.5 W/cm.sup.2 average power and 5.0.times.10.sup.8 W/cm.sup.2 peak power at wavelengths of 512 and 578 nm. However, it is noted that the damage threshold varies between fibers depending on the presence of impurities, the OH.sup.- content of the glass, the wavelength of the laser as well as a variety of other variables. High damage threshold surfaces are surfaces enabling power to be transmitted through the fiber at levels near the damage threshold of the fiber.
While high damage threshold surfaces can be achieved by carefully polishing the fiber face, it is generally the case that properly cleaved fibers produce fiber faces with higher damage thresholds. Cleaving results in fewer subsurface defects and does not lead to contamination of the fiber face by the lapping compounds. Therefore, cleaving of optical fibers is preferred over polishing as a method for generating high quality fiber face surfaces.
Optical fibers are generally cleaved by first scribing the fiber using a suitable scribing means which is commonly made of tungsten carbide or diamond. Scribing consists of placing the sharp edge of the scribing means at the desired cleavage point, holding the scribing means perpendicular to the fiber and gently drawing the scriber across the fiber. If desired, the fiber can be scribed around the full diameter of the fiber. The small scratch created by the scriber disrupts the molecular bonds of the glass and provides a starting point from which the glass will fracture.
Once the fiber has been scribed, the fiber is most commonly cleaved through the application of tension along the length of the fiber. Fibers smaller than 200 .mu.m in diameter can be cleaved by hand with good quality cleaves. However, fibers greater than 200 .mu.m in diameter require more tension and are sensitive to the alignment of the pull. Thus, more rigid tensioning methods are needed to achieve high quality cleaves for fibers having diameters in excess of 200 .mu.m.
In addition to applying tension to the scribed fiber by tension forces, several devices have been developed that apply torque forces at the scribed point on the fiber instead of tension forces. U.S. Pat. No. 3,934,773; Great Britain Patent No. 2021094; Danish Patent No. 3126852; Danish Patent No. 30135645; Danish Patent No. 2631678; Japanese Patent No. 57/178203; Japanese Patent No. 57/108802; Japanese Patent No. 57/8504. Japanese Patent No. 57/24903 espouses the combined use of tension forces with torque forces to cause fiber fracture. A variation of this device is also put forth wherein the fiber is scribed after tension is first applied. L. C. Mandigo (1979) "Fibre-Optic Cutting Tool," IBM Technical Disclosure Bulletin, 22:5 1787; GB 2118539.
Optical fibers have also been cleaved by spark erosion and by thermal shock. See Optical Fibres, Chapter II, "Fiber End Preparation", (Geisler, Beaven and Boutruche, eds.) Weaton & Co., Ltd., Great Britain p. 190 (1986) and references cited therein.
It is important to note that the above disclosed devices for cleaving optical fibers have not been applied to fibers with core diameters greater than 400 .mu.m. Cleaving of fibers greater than 400 .mu.m requires that the scribing means and the tensioning means be very precisely aligned. To date, no cleaving device has been produced that can be adequately aligned so as to enable the satisfactory cleaving of fibers larger than 400 .mu.m.
At present, a simple, quick and inexpensive device and method for cleaving optical fibers greater than 400 .mu.m is needed.